An interesting aspect about yeast is the preference it has for types of sugar. Most yeast prefer glucose so they are what is called glucophilic. The most common fermentation yeast, Saccharomyces cerevisiae, used for wine, beer, and bread is generally glucophilic. While many strains are capble of processing glucose snd fructose, some are highly glucophilic and will not process fructose efficiently. In order to complete fermentation, wine makers may need to add a fructophilic strain to the fermentation process. Fructophilic means the yeast prefers fructose as its primary sugar. This highlights one of the key differences between wine and cider. Besides having much higher levels of total sugars, grape wines contain more glucose versus fructose. Apples on the other hand, contain more fructose.
One of the key genes in yeast that manage the sugar uptake and processing are the HXT genes. Yeast that are fructophilic also have FRT genes that support the transport of fructose into the yeast cells. Starmerella bacillaris, previously known as Candida zemplinina, is a example of a yeast that is fructophilic. It’s a yeast commonly found on grapes, apples, and even prickly pear fruit. Like most non-Saccharomyces yeasts, it was incorrectly considered a spoilage yeast and was sought to be suppressed. We are now understanding the important role non-Saccharomyces yeast can play in the fermentation, stabilization, and quality of wine and cider. It’s the cider part that made me pursue trialing a couple strains from the USDA. Given the level of fructose in apples, it seems like we should be exploring more yeasts that are fructophilic.
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